Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit of a fine pattern on e.g. a Si substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices has been accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF laser (wavelength: 193 nm) is employed, about 45 nm is presumed to be the limit. From this point of view, EUV lithography, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser, is expected to be prospective as an exposure technique for 45 nm or below. In this specification, EUV light means a light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically a light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive indices of substances at such a wavelength are close to 1, whereby it is not possible to use a conventional dioptric system like photolithography employing visible light or ultraviolet light. For this reason, for EUV lithography, a catoptric system, i.e. a reflective photomask (hereinafter also referred to as “EUV mask”) and a mirror, is employed.
A mask blank is a stacked structure for a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, are formed in this order on a substrate made of e.g. glass (Patent Document 1). Besides these layers, such an EUV mask blank usually has a protective layer for protecting the reflective layer at the time of forming a mask pattern in the absorber layer, between the reflective layer and the absorber layer. Further, a low reflective layer is usually formed on the absorber layer for improving contrast at the time of inspection of the mask pattern.
In an EUV mask blank, the thickness of the absorber layer is preferably thin. In EUV lithography, exposure light is not incident from a perpendicular direction to an EUV mask but incident from a direction at an angle of a few degrees, usually 6 degrees, to the perpendicular direction. If the thickness of the absorber layer is thick, at the time of EUV lithography, a shadow by the exposure light arises on a mask pattern formed by removing a part of the absorber layer by etching, and the form accuracy or the dimension accuracy of a mask pattern (hereinafter referred to as “transfer pattern”) transferred to a resist on a substrate such as a Si wafer by using the EUV mask, tends to be deteriorated. Since this problem becomes more significant as the line width of the mask pattern formed on the EUV mask becomes smaller, the thickness of the absorber layer of the EUV mask blank is required to be thinner.
Ideally, a material having a high extinction coefficient for EUV light should be employed for the absorber layer of the EUV mask blank, and the thickness should be such that EUV light incident into a surface of the absorber layer is completely absorbed. However, as described above, since the thickness of the absorber layer is required to be thin, it is not possible for the absorber layer to completely absorb EUV light incident into the layer, and a part of the light becomes reflected light.
What is required when a transfer pattern is formed on a resist formed on a substrate by EUV lithography is the contrast of reflected light from the EUV mask, that is, the contrast between reflected light from a portion of the mask wherein the absorber layer is removed at the time of forming the mask pattern so that the reflective layer is exposed to the outside, and reflected light from a portion of the mask wherein the absorber layer is not removed at the time of forming the mask pattern. Accordingly, it has been considered that so long as a sufficient contrast of reflected light is secured, there is no problem even if the incident EUV light is not completely absorbed by the absorber layer. Based on the above concept, Patent Document 2 proposes an EUV mask using the principle of phase shift to reduce the thickness of the absorber layer.
However, in the above principle and the layer construction, although there is no problem for a real mask pattern region (a region wherein a mask pattern is formed, which is used for pattern-transferring at the time of EUVL), there is a problem for a region outside the pattern area. This point will be hereinafter described with reference to FIG. 7. FIG. 7 is a schematic cross-sectional view of an example of EUV mask after a pattern is formed, wherein a substrate 120 has a reflective layer 130 and an absorber layer 140 in this order thereon, and in a mask pattern region 210, a mask pattern formed by partially removing the absorber layer 140, is present. In the mask pattern region 210 of an EUV mask 100, by the above principle of phase shift, a reflection contrast between a surface of the reflective layer 130 and a surface of the absorber layer 140 can be sufficiently maintained. However, the region really irradiated with EUV light is a real exposure region 200. Accordingly, a region 220 outside the mask pattern region 210 is also irradiated with EUV light. At this time, since the effect of phase shift with reflected light from the reflective layer 130 is not sufficiently obtained, a resist on a Si substrate is irradiated with EUV light from the surface of the absorber layer 140, and there may occur a problem of unnecessary exposure of resist. Particularly, at the time of carrying out an overlay exposure, this problem becomes significant.
In order to solve such a problem, Patent Document 3 proposes a structure wherein an absorber film (a second absorber film) which absorbs EUV light is newly stacked outside the mask pattern region 210. In Patent Document 3, a Cr film is formed as a second absorber film.
Further, Patent Document 4 proposes forming a groove-like light-shielding band by removing a reflective layer 130 so as to contact and surround the outer circumference of the mask pattern region 210.
Among such methods, if the method of forming a light-shielding band is employed, the film stress of the EUV mask may be loosened by removing the reflective layer, and the position accuracy may be deteriorated, and accordingly, the method of stacking a film which absorbs EUV light, outside the mask pattern region 210 is more preferred.